1. Field of the Invention
The present invention generally relates to an apparatus for measuring a surface profile by the utilization of interatomic forces acting between a probe and a surface to be measured and, more particularly, a displacement detector adapted to measure a minute displacement of the probe over a distance on the order of a nanometer, which would result from a change in interatomic force acting between the probe and the surface, and also for the measurement of a minute displacement of the probe on a stroke in the order of micrometer.
2. Description of the Prior Art
As a means for measuring a minute surface profile, the method has been well known in the art, which comprises the steps of detecting an interatomic force acting between a sample surface to be measured and a sharp-pointed probe spaced a slight distance from the sample surface, moving the sample surface relative to the probe in a direction perpendicular to the probe, while the probe and the sample surface are spaced from each other a distance in the order of nanometer so that the detected interatomic force can attain a constant value, and scanning the position of the sample surface in a direction parallel to the probe thereby to accomplish a surface profile measurement. According to this known method, the interatomic force acting between the probe and the sample surface is required to be of a constant value in order for the distance between the sample and the probe to be maintained at a constant value and, for the detection of this interatomic force, the amount of deviation of the probe, supported by a spring, which would occur under the influence of the interatomic force is detected to determine the distance between the probe and the sample surface. In this type of surface profile measuring apparatus, demands have been made for it to have a capability of measuring the displacement of the probe to the accuracy of a nanometer or smaller.
The prior art displacement detector embodying the above described method will now be discussed with reference to FIG. 3 of the accompanying drawings.
Referring to FIG. 3, the prior art displacement detector comprises a polarizing beam splitter 101, .lambda./4 wavelength plates 102 and 103, an optical lens 104, a probe 105 supported by a holder in a cantilever fashion and having a displacement plane, a mirror 106 defining a reference plane, polarizing plates 107 and 110, sensors 108 and 111 and a beam splitter 109.
The prior art displacement detector of the above described construction operates in the following manner.
Laser beams of first and second frequencies slightly different from each other, which have been linearly polarized in respective directions perpendicular and parallel to the plane of the drawing enter the beam splitter 109. The laser beams entering the beam splitter 109 are divided into first and second laser beam components. The first laser beam components of the laser beams of the first and second frequencies travel towards the polarizing plate 110 through which only respective beam components polarized at 45 degrees relative to the plane of the drawing are allowed to pass therethrough. The polarized beam components enter the photosensor 111 which subsequently generates a reference signal indicative of a beat frequency between the first and second frequencies.
The second laser beam components emerging from the beam splitter 101 travel towards the polarizing beam splitter 101 and are divided by the polarizing beam splitter 101 into first, second and third polarized beam components. The first polarized beam component, polarized in a direction parallel to the plane of the drawing, travels, as a reference beam, towards the mirror 106 through the .lambda./4 wavelength plate 103 and is then reflected by the mirror 106 so as to travel backwardly towards the photosensor 108 again through the .lambda./4 wavelength plate 103, then through the polarizing beam splitter 101 and finally through the polarizing plate 107. During the passage of the first polarized beam component, which has been reflected by the mirror 106, through the .lambda./4 wavelength plate 103, the first polarized beam component is polarized at an angle of 90 degree.
The second polarized beam component, polarized in a direction perpendicular to the plane of the drawing during the passage thereof through the polarizing beam splitter 101, is directed, as a displacement beam, towards the probe 105 through the .lambda./4 wavelength plate 102 and then through the lens 104 and is subsequently reflected by the displacement plane of the probe 105 so as to travel backwardly towards the polarizing beam splitter 101 through the lens 104 and then through the .lambda./4 wavelength plate 102. The second polarized beam component reflected from the displacement plane of the probe 105 is polarized 90 degree during the passage thereof through the .lambda./4 wavelength plate 102 and subsequently enters the polarizing beam splitter 101 which serves to reflect it so as to travel towards the photosensor 108 through the polarizing plate 107.
The beat due to the difference in frequency between the reference beam reflected by the mirror 106 and the displacement beam reflected by the probe 105 is detected by the photosensor 108. When the probe displaces, the length of the optical path for the displacement beam varies correspondingly and the difference in length between the optical path for the displacement beam and that for the reference beam is outputted in the form of a change in phase of a beat signal detected by the photosensor 108. By determining the difference in phase between the beat signal and the beat signal detected by the photosensor 111, the amount of displacement of the probe can be detected.
The prior art displacement detector of the above described construction has a problem in that, since the position used as a reference for the measurement of the position of the probe 105 is represented by the mirror 106, an erroneous detection in position of the probe tends to occur when the mirror 106 vibrates under the influence of, for example, an external disturbance. It also has another problem in that, since the optical paths along which the reference beam and the displacement beam travel, respectively, differ from each other, the measurement is apt to be adversely affected by a change in ambient temperature and/or ambient pressure and also by a change in refractive index resulting from a change in temperature.